Non-woven fabric treatment

ABSTRACT

Non-woven fabrics are disclosed comprising substantially unbonded fibers and vinyl monomers which are capable of reacting with an acid or a base to form a salt directly or indirectly by a reaction involving exposure to ultraviolet radiation while impregnated with a solution of the vinyl monomer copolymerized to the surface of the fibers. Laminates of these non-woven fabrics as well as electrochemical devices employing them as a separator, and methods of treating such non-woven fabrics are also disclosed.

[0001] This invention relates to a method of treating a non-woven fabricand to a treated non-woven fabric. The fabric can be used as a separatorin an electrochemical device.

[0002] Non-woven fabrics are used for separation applications relying onthe porosity that is available from the non-woven structure. A balancehas to be established between appropriate porosity and the physicalproperties of the fabric, in particular in terms of tensile strength ofthe fabric. The material and structure of the fabric have to be selectedaccording to the requirements for the fabric when in use. An example ofan application for a non-woven fabric is as an electrode separator in anelectrochemical device such as an electrochemical cell. Examples of suchcells include nickel-cadmium and nickel-metal hydride cells. Theseparator should be inert towards materials with which it comes intocontact in the cell including in particular the alkaline electrolyte andthe electrode materials. It should also have physical characteristicswhich enable it to withstand the treatment encountered during assemblyof the device and during use. For example, it should be able towithstand the stresses encountered during spiral winding of the cellcomponents. It should also be capable of resisting the growth ofdendrites between the electrodes during recharging.

[0003] Fabrics formed from polypropylene fibres have appropriateproperties for use as separators in many electrochemical devices.

[0004] Non-woven fabrics can be made by processes which include (a) meltblowing, (b) spinning, and (c) wet or dry laying. The fibres of fabricsmade by spinning and wet or dry laying require bonding to one anotherfor the fabric to have integrity, so that it has the mechanicalproperties required for satisfactory performance. In the case of fabricsmade by spinning, the fibres are bonded to one another by theapplication of heat and pressure so that the structure of the fabric isstable. In the case of fabrics made by wet or dry laying frompolypropylene fibres, polyethylene is incorporated into the fabric,either as fibres consisting just of polyethylene or as bicomponentfibres consisting of a polypropylene core and a polyethylene sheath. Thepolyethylene in the fabric can provide the necessary bonds as a resultof heating the fabric to a temperature that is greater than thesoftening point of the polyethylene.

[0005] A fabric that is made from spun fibres which are then bondedtogether (a “spun bonded” fabric) can have the disadvantage that thebonds reduce the effective surface area of the fabric that is availableto ion transfer by effectively blocking the pores of the fabric. Theuneven current distribution that results from this uneven poredistribution can give rise to dendrite formation during recharging of asecondary cell, ultimately leading to a short circuit in the cell. Thereis therefore a compromise to be reached with such fabrics betweenmechanical properties that are enhanced by bonds between the fibres andelectrochemical performance which is diminished by the bonds.

[0006] A fabric that is formed by wet or dry laying of fibres hassatisfactory mechanical properties. However, especially when bicomponentfibres are used, the fibre size can tend to be undesirably large, oftengreater than 15 μm.

[0007] An example of a process for treating a polyolefin non-wovenfabric to render it hydrophilic is disclosed in WO-A-93/01622. Theprocess involves impregnating a non-woven fabric formed from polyolefinfibres with an aqueous solution of a vinyl monomer (such as acrylicacid), and exposing the impregnated fabric to ultraviolet radiationwhile restricting exposure of the fabric to oxygen. The process resultsin co-polymerisation of the vinyl monomer and the polyolefin of thefibres. Surprisingly, it also results in crosslinking of the material ofthe fabric. This can therefore result in an improvement in the tensileproperties of the fibres. The resulting treated fabric can also be foundto exhibit good resistance to degradation on exposure to alkalineelectrolyte materials.

[0008] The technique disclosed in WO-A-93/01622 has been found to havebeneficial effects on the physical properties of the polymeric materialof the fibres of a non-woven fabric. However, acceptable physicalproperties of the separator have hitherto been derived from bondsbetween the fibres of the separator, through the application of heat andpressure or by incorporation of additional bonding materials or both.

[0009] The present invention provides a technique for treating anon-woven fabric involving copolymerisation of a vinyl monomer to thesurface of the fibres by an ultraviolet radiation initiated reactionwhich, applied to a fabric formed from substantially unbonded fibres,has been found to improve the physical properties of the fabric as wellas render it hydrophilic.

[0010] Accordingly, in one aspect, the invention provides a non-wovenfabric which is formed from fibres which are substantially unbonded andwhich has copolymerised to the surface of the fibres a vinyl monomerwhich is capable of reacting with an acid or a base to form a saltdirectly or indirectly by a reaction which involves exposure of thefabric to ultraviolet radiation while impregnated with a solution of thevinyl monomer.

[0011] In another aspect, the invention provides a method of treating anon-woven fabric which comprises:

[0012] (a) impregnating the non-woven fabric with a solution of a vinylmonomer capable of reacting with an acid or a base to form a saltdirectly or indirectly, the solvent being one which does not evaporatesignificantly in the subsequent step of exposing the fabric toradiation, and

[0013] (b) exposing the impregnated fabric to ultraviolet radiationwhile the exposure of the fabric to oxygen is restricted, to cause themonomer and the material of the fibres to co-polymerise,

[0014] in which the structure of the fabric prior to the treatment issuch that the fibres are substantially unbonded.

[0015] The technique of the present invention has the advantage that itenables hydrophilic fabrics to be made from non-woven fabrics which, dueto their construction, have physical properties prior to the hydrophilictreatment which make them unsuitable for many applications because ofthe conditions to which such fabrics are exposed, prior to and duringuse. Such fabrics include those made by techniques such as spinning, andwet or dry laying, without a subsequent bonding step. It provides theadvantage of enabling a non-woven fabric to be made with enhancedphysical properties, without the disadvantages arising from the largesize of bicomponent fibres, and from disruption of the surface of afabric due to localised heat and pressure bonding. For example in anelectrochemical device, the use of a separator formed from a non-wovenfabric with fine fibres can minimise the internal resistance of thedevise, and can extend the recharging cycle life of the device as aresult of good absorption and subsequent retention of electrolyte.

[0016] The improvement in physical properties that can be obtained innon-woven fabrics from the technique of the present invention includeincreases in tensile strength of the fabric, measured in its machinedirection. The tensile strength can be increased as a result of thecopolymerisation reaction, compared with the tensile strength prior tothe reaction, by at least about 50%, preferably at least about 100%,more preferably at least about 150%, for example at least about 200%.Importantly, these increases in tensile strength can be obtained atlevels of the copolymerisation reaction which provide acceptablehydrophilic properties but without such high levels of graftpolymerisation that the fabric is caused to swell excessively.Accordingly, the ratio of the tensile strength of the fabric measured inthe machine direction after the copolymerisation reaction to that of thefabric prior to the reaction can be at least about 1.5, preferably atleast about 2.0, especially at least about 3.0.

[0017] Surprisingly, significant increases in tensile strength can beachieved by the technique of the present invention in non-woven fabricsin which the fibres are substantially unbonded, whereas copolymerisationof vinyl monomer reaction with the fibres of a non-woven fabric made bytechniques such as spinning or wet or dry laying, with subsequentbonding, can only give rise to small increases in tensile strength. Forexample, for equivalent extents of the copolymerisation reaction,increases in tensile strength of as much as 300% can be obtained infabrics of unbonded fibres (for example a dry laid fabric), but only ofabout 40% in fabrics of bonded fibres (for example a spun bondedfabric).

[0018] Examples of fabrics in which the fibres are substantiallyunbonded are fabrics formed from spun fibres and fabrics formed fromlaying fibres, either wet or dry, without a subsequent step of bondformation by the application of heat and pressure. There might be weakforces between the fibres of such fabrics. For example, weak forces canresult from a step of calendering a fabric under moderate heat andpressure, which can lead to localised deformation of the fibre material,especially where fibres come into contact with one another. However, theforces will be capable of being overcome when the fabric is placed undertension. It will be possible to discern a boundary between the fibres ofthe fabric. There will not be any intimate mixing of the materials ofthe fibres as results from the formation of a weld. Calendering thefabric after the graft polymerisation reaction has been found to giverise to enhanced electrolyte absorption. A fabric that has beencalendered after the graft reaction can have an improved ability toabsorb impurities, especially ammonia, which might be present in theelectrolyte system. Moreover, fibres of the fabric are less likely to bedamaged physically as a result of the calendering step when it iscarried out after the graft polymerisation reaction.

[0019] The fibres of the fabric will be formed from a polymeric materialwhich is inherently hydrophobic and which is capable of undergoing thepolymerisation reaction with the vinyl monomer on its surface. Thereaction can render the fabric wettable to aqueous media. The fibres caninclude polymers such as for example polyamides, polyesters andnaturally occurring materials such as cellulose based materials.Preferred polymeric materials are polyolefins such as polyethylenes andpolypropylenes.

[0020] Preferably, the material of the surface of at least some of thefibres, for example at least about 40% by weight, preferably at leastabout 60%, more preferably at least about 80%, comprises polypropylene.Preferably, at least 40% by weight of the material of the fibres of thefabric is polypropylene, more preferably at least about 60%, especiallyat least about 80%.

[0021] Preferably, the material of at least some of the fibres fromwhich the first or second fabric (or each of the fabrics) is formed, forexample at least about 40% by weight, preferably at least about 60%,more preferably at least about 80%, is substantially homogeneousthroughout the thickness of the fibres. It can be preferred for manyapplications for the material of substantially all of the fibres to besubstantially homogeneous throughout their thickness, so that thosefibres are formed only from polypropylene or another suitable material(with appropriate additives where necessary).

[0022] The fabric can be made from fibres comprising more than onematerial, for example more than one polymer or a polymer havingdifferent physical properties in different regions of the fibres or thefabric. For example, the fabric may be made from at least some fibresformed from two polymers such as bicomponent fibres with the componentsarranged coaxially or side-by-side.

[0023] It is particularly preferred that the fabric is formed fromfibres which comprise polypropylene alone. This has the advantage thatthe physical properties of the fabric are those of a non-woven fabricformed from polypropylene fibres which are generally preferred comparedwith other polyolefin fibres. Compared with bicomponent fibres, the usejust of polypropylene fibres has the advantage that the fibres can bemade thin without increasing the cost undesirably.

[0024] The ion exchange capacity of the polymeric sheet is measured inmeq.g⁻¹ according to the test routine referred to below, to provide ameasure of the extent of the graft copolymerisation reaction between thematerial of the fibres and the vinyl monomer. Preferably, the ionexchange capacity is at least about 0.15, more preferably at least about0.4, especially at least about 0.6. Preferably, the ion exchangecapacity is not more than about 2.0, more preferably not more than about1.6, especially not more than about 1.4, for example not more than about1.2. It has been found that useful increases in the physical propertiesof polypropylene fibres of a non-woven fabric and of the fabric itselfformed from unbonded fibres can be obtained at low graft levelscorresponding to these values of the ion exchange capacity.

[0025] The gel fraction of the material of the fabric is measuredaccording to ASTM D2765-84, providing a measure of the extent ofcrosslinking of the fabric. Preferably, the gel fraction is at leastabout 10%, more preferably at least about 20%, especially at least about30%.

[0026] Preferably, the mean thickness of the fibres (which might bemeasured as a mean diameter, especially when the fibres have a circularcross-section) from which the non-woven fabric is formed is less thanabout 30 μm, more preferably less than about 10 μm. The thickness of thefibres will often be more than about 5 μm.

[0027] Preferably, the effective mean size of pores that are defined bythe fibres of the fabric, as measured using a Coulter porometer, is lessthan about 60 μm, more preferably less than about 45 μm, for exampleless than about 30 μm. Such small pore sizes can be attained using smalldiameter fibres, such as those referred to above. The use of a fabricwith a small pore size as a separator in an electrochemical device hasthe advantage that the ability of the separator to prevent penetrationof electrode materials, for example as dendrites, is enhanced. A smallpore size also enhances the ability of the fabric to absorb and toretain electrolyte once the fibres have been treated to render themhydrophilic. A high electrolyte absorption has the advantage of reducingthe internal resistance of a device in which the fabric is incorporatedas an electrode separator, and of extending the cycle life of thedevice.

[0028] Preferably, the thickness of the fabric, measured using testmethod DIN 53105 which involves lowering a 2.0 kg weight onto a sampleof the sheet of area 2.0 cm² at a speed of 2.0 mm.s⁻¹, is greater thanabout 80 μm, more preferably greater than about 100 μm; preferably, thethickness is less than about 400 μm, more preferably less than about 250μm. The method by which the sheet is made may include a step ofcalendering the fabric to reduce its thickness to a value within therange referred to above, the reduction being by at least about 5%,preferably at least about 15%, more preferably at least about 25%, andless than about 60%, preferably less than about 45%, more preferablyless than about 40%. Calendering can have the advantage of reducing theeffective size of the pores in the fabric, improving its barrierproperties. The calendering step may take place before or after thematerial of the fabric is reacted with the graft-polymerisationsolution. Calendering the fabric before the graft-polymerisationreaction has been found to give rise to increased rates of the reaction.

[0029] The vinyl monomer which is graft-polymerised with thepolypropylene of the fibre surface can be capable of reacting with anacid or a base directly to form a salt, or indirectly to form a saltafter appropriate work up, perhaps involving for example hydrolysis orsulphonation. Preferred vinyl monomers include ethylenically unsaturatedcarboxylic acids and esters thereof such as acrylic acid, methacrylicacid, methyl acrylate, and methylmethacrylate. Other vinyl monomerswhich might be used include acrylamide, vinylpyridine, vinylpyrrolidoneand styrene-sulphonic acid.

[0030] In another aspect, the invention provides a laminate of a fabricas discussed above and at least one further non-woven fabric formed fromfibres of a hydrophobic polymeric material which has undergone acopolymerisation reaction with a vinyl monomer which is capable ofreacting with an acid or a base to form a salt directly or indirectly bya reaction. The fabrics of the laminate can be bonded to one another.However, they can be unbonded for some applications. Features oflaminates of non-woven fabrics which can be incorporated into thelaminate of the present invention are disclosed in the patentapplication filed with the present application, claiming priority fromUK patent application no. 9712692.4 and entitled NON-WOVEN FABRICLAMINATE (bearing the agents' reference P10600). Subject matterdisclosed in the specification of that application is incorporated inthe present specification by this reference.

[0031] In a further aspect, the invention provides an electrochemicaldevice, comprising an anode, a cathode, a quantity of an electrolyte,and an electrode separator formed from a fabric of the type discussedabove. Preferably, the cathode in the device comprises nickel (II)hydroxide. An example of material which can form the anode in such adevice includes cadmium. Alternatively, the anode may be a metal hydrideelectrode. Other types of electrochemical device in which the separatorof the invention finds application include secondary cells such aslead-acid cells.

[0032] The use of a solvent which does not evaporate to a significantdegree in the irradiation step of the method has been found to conferthe advantages of providing greater uniformity of properties of theresulting sheet, throughout the thickness of the sheet. Thus there isgreater uniformity in the degree of grafting throughout the thickness ofthe sheet, leading to improved ion exchange properties through thesheet. It is believed that this might arise at least in part because ofthe transparency of the sheet which is retained as a result of theretention of the solvent in the pores of the fabric. It has also beenfound that the degree or adverse effects or both of homopolymerisationof the vinyl monomer can be reduced by selection of an appropriatesolvent.

[0033] Suitable solvents for use in the method of the invention willgenerally be transparent to ultraviolet radiation, have no atoms whichare abstractable when exposed to radiation, have a high specific heatand a high latent heat of vaporisation, and will not react adverselywith the material of the fibres of the separator. Preferred solventswill have a boiling point which is greater than about 50° C., preferablygreater than about 70° C. It is also preferred that the boiling point ofthe solvent be no higher than a temperature at which the film might bedamaged during the course of the irradiation step of the method. Forexample, the boiling point of the solvent might be selected to be lessthan the temperature at which the material of the fibres melts orsoftens. Particularly preferred solvents have a latent heat ofvaporisation which is greater than about 1000 J.g⁻¹, preferably greaterthan about 1500 J.g⁻¹, more preferably greater than about 2000 J.g⁻¹,and/or a specific heat capacity which is greater than about 2.0J.g⁻¹.K⁻¹, preferably greater than about 3.0 J.g⁻¹.K⁻¹, more preferablygreater than about 4.0 J.g⁻¹.K⁻¹. A value of specific heat capacity, orof latent heat of vaporisation, within these ranges has the advantagethat the solvent in the reaction has an enhanced ability to dissipateheat without evaporating to a significant degree, giving rise to theadvantages referred to above. A particularly significant furtheradvantage is that the formation of product from the homopolymerisationreaction of the vinyl monomer is restricted, and any such product whichis formed is retained in solution rather than being deposited in thepores within the sheet. This allows the product to be removed easilyfrom the sheet by washing. The control over the formation of thehomopolymerisation product can be achieved without use of inhibitingagents, which can cause contamination problems when the sheet is in usein certain applications. Water is a particularly preferred solvent.

[0034] The ultraviolet radiation initiated polymerisation reaction canbe completed surprisingly quickly, for example by exposing theimpregnated fabric to radiation for as little as 15 seconds, even aslittle as 5 or 10 seconds, and it has been found that the fabric afterreaction contains a significant amount of grafted monomer, which can besufficient for the fabric to be rendered wettable by aqueous solutionssuch as might be found in certain electrochemical devices.

[0035] Techniques by which exposure of the impregnated fabric to oxygencan be restricted include, for example, carrying out the ultravioletirradiation step in an inert atmosphere, for example in an atmosphere ofargon or nitrogen, or sealing the impregnated fabric between sheets ofmaterial which are impervious to oxygen, but are transparent toultraviolet radiation of appropriate wavelength for initiating thecopolymerisation reaction.

[0036] Preferably, the impregnation solution includes an initiator forthe polymerisation reaction. Preferably, the initiator initiates thereaction by abstracting an atomic species from one of the reactingmaterials, for example by abstracting a hydrogen atom from thepolypropylene of the fabric fibres to create a polymeric radical.Following such abstraction, the polymeric radical, in contact with themonomer in solution, can initiate the formation of a grafted branch.When an atom is abstracted from the polypropylene of the fabric fibres,the activated polypropylene molecule can react either with anotherpolypropylene molecule so that the polypropylene of the fabric becomescross-linked, or with the vinyl monomer in a co-polymerisation reaction.An example of a suitable initiator is benzophenone. The mole ratio ofthe vinyl monomer to the initiator is preferably at least about 50, morepreferably at least about 100, especially at least about 175; the ratiois preferably less than about 1500, more preferably less than about1000, especially less than about 500, more especially less than about350; for example the ratio may be about 200.

[0037] The impregnation solution may include a component by whichhomopolymerisation of the vinyl monomer is inhibited. Examples ofsuitable inhibitors include iron (II) and copper (II) salts which aresoluble in the reaction medium, a preferred material for aqueous mediabeing iron (II) sulphate. It has been found, however, that the need foran inhibitor can be avoided by selection of an appropriate solvent forthe graft-polymerisation reaction which can restrict the speed anddegree of the homopolymerisation reaction, for example as a result ofits ability to act as a heat sink. This can be an advantage when it isdesired to minimise the amount of contaminants in the sheet.

[0038] The impregnation solution may include additional components tooptimise reaction conditions such as surfactants to ensure that thesolution fully impregnates the non-woven fabric, an appropriate mixtureof solvents to ensure homogeneity of the solution, and so on.

[0039] The use of ultraviolet radiation in the method of the inventionallows non-woven fabrics suitable for use as electrode separators to bemade economically and on a continuous basis. It has been found thatsufficient energy can be supplied to an impregnated fabric for theirradiation process to be run continuously, and that the heat which isgenerated in such a process can be controlled by use of appropriatesolvents as heat sink components.

[0040] A benefit of the present invention is that physical properties ofthe treated fabric (in particular, its tensile strength or its abilityto be wetted by aqueous solutions or both) can be stable on prolongedexposure to an alkaline solution. A fabric with stable physicalproperties is particularly appropriate for use as a separator inelectrochemical devices in which the electrolyte comprises an alkalinesolution. A test to determine stability on exposure to alkaline solutioninvolves storing a sample of a fabric to a solution containing 30% byweight of potassium hydroxide at 71° C. for 21 days, and then comparingthe selected property of the exposed fabric to that of a fabric that hasnot been exposed to the alkaline solution.

[0041] Measurement of Ion Exchange Capacity

[0042] A sample of membrane about 0.5 g is converted into the acid (H⁺)form by immersion in 1.0 M hydrochloric acid at 60° C. for 2 hours. Thesample is washed in distilled water until the washing water shows a pHin the range of about 6 to 7. The sample is then dried to constantweight at 70° C.

[0043] The dried sample is placed in a 100 ml polyethylene bottle towhich is added accurately 10 ml of approximately 0.1 M potassiumhydroxide. Additional distilled water can be added to immerse the samplefully. A further 10 ml of potassium hydroxide is added to a secondpolyethylene bottle, together with the same amount of distilled water asthat added to the bottle containing the sample. Both bottles are storedat 60° C. for at least two hours.

[0044] After being allowed to cool, the contents of each bottle aretransferred to glass conical flasks, and the amount of potassiumhydroxide in each is determined by titration with standardised 0.1 Mhydrochloric acid, using a phenolphthalein indicator.

[0045] The ion exchange capacity, measured in milliequivalents per gram,of the membrane in the dry acid (H⁺) form is calculated according to theequation: ${IEC} = \frac{t_{2} - t_{1}}{10W}$

[0046] where t₁ is the titration value of HCl from bottle with thesample, t₂ is the titration value of HCl from bottle without the sample,and W is the weight of the dried membrane in acid (H⁺) form.

[0047] Examples of treatments of non-woven fabrics are set out below.

EXAMPLE 1

[0048] A dry laid non-woven fabric with a thickness of 140 μm and abasis weight of 60 g.m⁻² was manufactured from carded pure polypropylenestaple fibres. A web formed from fibres was densified by passage througha set of smooth rollers which were heated to a temperature of 160° C.

[0049] The fabric was immersed in a solution formulated as follows(percentages by weight): Component wt. % Acrylic acid 30.0 Benzophenone0.25 Surfactant (Lutensol ON70 ™) 0.5 Water 69.25

[0050] The impregnated fabric was maintained in an atmosphere ofnitrogen and passed through an irradiation chamber defined by quartzglass walls. Medium pressure mercury vapour lamps were positionedparallel to one another on opposite sides of the chamber outside thequartz glass walls. The lamps had a power output of 120 W.cm⁻¹ and werelocated 16 cm from the fabric. Each lamp provided a parallel ultravioletlight beam with a width of 10 cm. The total exposure time of the fabricto the radiation was about 6 seconds.

[0051] The fabric was then washed in de-ionised water to removeunreacted components and then dried in an air oven at approximately 70°C.

[0052] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g¹) 00.72 Gel content (%) (ASTM D2765-84) 0 56.1 Machine direction tensilestrength (N.m^(−1) (ASTM D 882)) 490 1584 Machine direction elongation(%) (ASTM D 882) 7.9 14.3 Electrolyte wicking rate (time) 60 s 600 s 60s 600 s (30% w/w KOH) (DIN 53924-78) (mm) 0^(a) 0^(a) 35 92 Electrolyteabsorbtion (%) (AD 447301 US Air Force Manual) Non-wetting^(a) 215

EXAMPLE 2

[0053] The procedure of Example 1 was followed with a dry laid non-wovenfabric with a thickness of 171 μm and a basis weight of 60 g.m⁻².

[0054] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g⁻¹) 00.2 Gel content (%) 0 45.4 (ASTM D2765-84) Machine direction tensile 4601660 strength (N.m⁻¹) (ASTM D 882)

EXAMPLE 3

[0055] A non-woven fabric with a thickness of 140 um and a basis weightof 45 g.m⁻² was manufactured from continuously spun pure polypropylenefibres. A web formed from fibres was densified by passage through a setof smooth bowl rollers which were heated to a temperature of 135° C.

[0056] The fabric was immersed in the acrylic acid solution andirradiated using the procedure described above in Example 1.

[0057] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g⁻¹) 00.75 Gel content (%) 0 78.3 (ASTM D2765-84) Machine direction tensile1050 3109 strength (N.m⁻¹) (ASTM D 882)

EXAMPLE 4

[0058] The procedure of Example 3 was followed with a dry laid non-wovenfabric with a thickness of 177 μm and a basis weight of 45 g.m⁻².

[0059] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g⁻¹) 00.7 Gel content (%) 0 88.4 (ASTM D2765-84) Machine direction tensile 8002641 strength (N.m⁻¹) (ASTM D 882)

EXAMPLE 5

[0060] The procedure of Example 1 was used to make a dry laid non-wovenfabric from a mixture of 50 wt. % pure polypropylene staple fibres and50 wt. % polyethylene/polypropylene bicomponent staple fibres. Thefabric had a thickness of 145 μm and a basis weight of 60 g.m⁻¹. It wasimmersed in an acrylic acid solution and irradiated using the proceduredescribed above in Example 1.

[0061] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g⁻¹) 00.7 Gel content (%) 0 56.1 (ASTM D2765-84) Machine direction tensile 4011115 strength (N.m⁻¹) (ASTM D 882)

[0062] Use in a Battery

[0063] An AA size alkaline spirally wound nickel-metal hydride (Mischmetal electrode) cell was constructed using a separator of the typedescribed above in Example 5. The cell was repeatedly charged at 350 mAand discharged through a 10 ohm passive load. The cell was found to becapable of delivering 1000 mA.h to a 1.0 V cut-off on discharge.

COMPARATIVE EXAMPLE

[0064] The procedure of Example 1 was followed with a spun-bondednon-woven polypropylene fabric used commercially to make batteryseparators, having a thickness of 200 μm and a basis weight of 50 g.m⁻².

[0065] The properties of the treated fabric are set out below, andcompared with the corresponding properties of the polypropylene fabricstarting material: Ungrafted Grafted Ion exchange capacity (meq.g⁻¹) 00.86 Gel content (%) 0 72.2 (ASTM D2765-84) Machine direction tensile3767 3880 strength (N.m⁻¹) (ASTM D 882)

1. A non-woven fabric which is formed from fibres which aresubstantially unbonded and which has copolymerised to the surface of thefibres a vinyl monomer which is capable of reacting with an acid or abase to form a salt directly or indirectly by a reaction which involvesexposure of the fabric to ultraviolet radiation while impregnated with asolution of the vinyl monomer.
 2. A fabric as claimed in claim 1, inwhich the ratio of the tensile strength of the fabric measured in themachine direction after the copolymerisation reaction to that of thefabric prior to the reaction is at least about 1.5.
 3. A fabric asclaimed in claim 1, in which the fabric has been produced by a processwhich involves any of wet laying, dry laying and fibre spinning.
 4. Afabric as claimed in claim 1 or claim 2, in which the mean thickness ofthe fabric is less than about 400 μm.
 5. A fabric as claimed in any oneof claims 1 to 3, in which the mean thickness of the fabric is at leastabout 80 μm.
 6. A fabric as claimed in any one of claims 1 to 5, inwhich the vinyl monomer comprises an ethylenically unsaturatedcarboxylic acid or an ester thereof.
 7. A fabric as claimed in any oneof claims 1 to 6, in which at least about 40% by weight of the materialof the fibres of the fabric is polypropylene.
 8. A fabric as claimed inany one of claims 1 to 7, in which the mean thickness of the fibres fromwhich the non-woven fabric is formed is less than about 30 μm.
 9. Alaminate of a fabric as claimed in any one of claims 1 to 8 and at leastone further non-woven fabric formed from fibres of a hydrophobicpolymeric material which has undergone a copolymerisation reaction witha vinyl monomer which is capable of reacting with an acid or a base toform a salt directly or indirectly by a reaction.
 10. An electrochemicaldevice comprising an anode, a cathode, a quantity of an electrolyte, andan electrode separator formed from a fabric as claimed in any one ofclaims 1 to 8 or a laminate as claimed in claim
 9. 11. A method oftreating a non-woven fabric which comprises: (a) impregnating thenon-woven fabric with a solution of a vinyl monomer capable of reactingwith an acid or a base to form a salt directly or indirectly, thesolvent being one which does not evaporate significantly in thesubsequent step of exposing the fabric to radiation, and (b) exposingthe impregnated fabric to ultraviolet radiation while the exposure ofthe fabric to oxygen is restricted, to cause the monomer and thematerial of the fibres to co-polymerise, in which the structure of thefabric prior to the treatment is such that the fibres are substantiallyunbonded. Please delete claims 1-11 and substitute therefor thefollowing new claims.
 12. A non-woven fabric comprising substantiallyunbonded fibers and a vinyl monomer which is capable of reacting with anacid or a base to form a salt directly or indirectly by a reaction whichinvolves exposure to ultraviolet radiation while impregnated with asolution of the vinyl monomer copolymerized to the surface of saidfibers.
 13. A fabric as claimed in claim 12, having a tensile strengthmeasured in the machine direction which is at least about 1.5 times thetensile strength of said fabric prior to said copolymerization reaction.14. A fabric as claimed in claim 12, produced by a process selected forthe gap consisting of wet laying, dry laying and fiber spinning.
 15. Afabric as claimed in claim 12, wherein the mean thickness of said fabricis less than about 400 μm.
 16. A fabric as claimed in claim 12, whereinthe mean thickness of said fabric is at least about 80 μm.
 17. A fabricas claimed in claim 12, wherein said vinyl monomer comprises anethylenically unsaturated carboxylic acid or an ester thereof.